Fastest Pulsar Speeding Out of Galaxy, Astronomers Discover

A speeding, superdense neutron star somehow got a powerful
"kick" that is propelling it completely out of our Milky Way
Galaxy into the cold vastness of intergalactic space. Its
discovery is puzzling astronomers who used the National Science
Foundation's
Very Long Baseline Array (VLBA) radio telescope
to directly measure the fastest speed yet found in a neutron star.

The
neutron star is the remnant of a massive star born in
the constellation Cygnus that exploded about two and a half
million years ago in a titanic explosion known as a
supernova.
Ultra-precise VLBA measurements of its distance and motion show
that it is on course to inevitably leave our Galaxy.

"We know that supernova explosions can give a kick to the
resulting neutron star, but the tremendous speed of this object
pushes the limits of our current understanding," said Shami Chatterjee,
of the National Radio Astronomy Observatory (NRAO) and the
Harvard-Smithsonian Center for Astrophysics. "This discovery
is very difficult for the latest models of supernova core
collapse to explain," he added.

Chatterjee and his colleagues used the VLBA to study the
pulsar
B1508+55, about 7700
light-years from Earth. With the ultrasharp
radio "vision" of the continent-wide VLBA, they were able to
precisely measure both the distance and the speed of the
pulsar,
a spinning neutron star emitting powerful beams of radio waves.
Plotting its motion backward pointed to a birthplace among
groups of giant stars in the constellation Cygnus -- stars
so massive that they inevitably explode as supernovae.

"This is the first direct measurement of a neutron star's speed
that exceeds 1,000 kilometers per second," said Walter Brisken,
an NRAO astronomer. "Most earlier estimates of neutron-star speeds
depended on educated guesses about their distances. With this
one, we have a precise, direct measurement of the distance, so
we can measure the speed directly," Brisken said. The VLBA
measurements show the pulsar moving at nearly 1100 kilometers
(more than 670 miles) per second -- about 150 times faster
than an orbiting Space Shuttle. At this speed, it could
travel from London to New York in five seconds.

In order to measure the pulsar's distance, the astronomers had
to detect a "wobble" in its position caused by the Earth's motion
around the Sun. That "wobble" was roughly the length of a baseball
bat as seen from the Moon. Then, with the distance determined,
the scientists could calculate the pulsar's speed by measuring
its motion across the sky.

"The motion we measured with the VLBA was about equal to
watching a home run ball in Boston's Fenway Park from a seat on
the Moon," Chatterjee explained. "However, the pulsar took
nearly 22 months to show that much apparent motion. The VLBA is
the best possible telescope for tracking such tiny apparent
motions."

The star's presumed birthplace among giant stars in the
constellation Cygnus lies within the plane of the
Milky Way,
a
spiral galaxy. The new VLBA observations indicate that the
neutron star now is headed away from the Milky Way's plane with
enough speed to take it completely out of the Galaxy. Since the
supernova explosion nearly 2 and a half million years ago, the
pulsar has moved across about a third of the night sky as seen
from Earth.

"We've thought for some time that supernova explosions can give
a kick to the resulting neutron star, but the latest computer
models of this process have not produced speeds anywhere near
what we see in this object," Chatterjee said. "This means
that the models need to be checked, and possibly corrected,
to account for our observations," he said.

"There also are some other processes that may be able to add
to the speed produced by the supernova kick, but we'll have to
investigate more thoroughly to draw any firm conclusions,"
said Wouter Vlemmings of the Jodrell Bank Observatory in the
UK and Cornell University in the U.S.

The observations of B1508+55 were part of a larger project to
use the VLBA to measure the distances and motions of numerous
pulsars. "This is the first result of this long-term project,
and it's pretty exciting to have something so spectacular come
this early," Brisken said. The VLBA observations were made at
radio
frequencies between 1.4 and 1.7
GigaHertz.

Chatterjee, Vlemmings and Brisken worked with Joseph Lazio of
the Naval Research Laboratory, James Cordes of Cornell University,
Miller Goss of NRAO, Stephen Thorsett of the University of California,
Santa Cruz, Edward Fomalont of NRAO, Andrew Lyne and Michael
Kramer, both of Jodrell Bank Observatory. The scientists
presented their findings in the September 1 issue of the
Astrophysical Journal Letters.

The
VLBA is a system of ten radio-telescope antennas, each with a
dish 25 meters (82 feet) in diameter and weighing 240 tons. From
Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S.
Virgin Islands, the VLBA spans more than 5,000 miles, providing
astronomers with the sharpest vision of any telescope on Earth
or in space.